Cyclobutene photochemistry. Adiabatic photochemical ring opening of alkylcyclobutenes

Abstract

The photochemistry of the cis and trans isomers of a series of dimethylbicyclo[n.2.O]alk-(n+2)-enes (n = 2-5) (bicyclic cyclobutene derivatives in which the C=C bond is shared by the two rings) in pentane solution is described. Irradiation of these compounds using monochromatic 193-or 214-nm light sources results in ring opening to yield the corresponding 1,2-bis(l-ethylidene)cycloalkanes (C,-C,) in high chemical and quantum yields. In all cases, the reaction proceeds with a high (70-90%) degree of disrotatory stereoselectivity. Quantum yields for direct cis,trans photoisomerization of the isomeric E,Eand E,Z-1,2-bis(l-ethylidene)cycloalkanes have also been determined. The product distributions from irradiation of the cyclobutenes are wavelength dependent, but for 214-nm excitation the isomeric diene distributions obtained from cyclobutene ring opening agree fairly closely with those calculated from the quantum yields for cis,trans photoisomerization of the isomeric dienes on the assumption that the process involves purely disrotatory ring opening to yield a single diene isomer in the lowest excited singlet state. The results are consistent with an orbital-symmetry-controlled, adiabatic mechanism for ring opening.Key words: photochemistry, cyclobutene, electrocyclic, adiabatic, conical intersection, orbital symmetry.Resume : On dCcrit la photochimie des isomkres cis et trans d'une sCrie de dimCthylbicyclo[n.2.0]alc-(n+2)-knes (n = 2-5) (dCrivts cyclobutknes bicycliques dans lesquels la liaison C% est partagCe entre deux cycles) en solution dans le pentane. L'irradiation de ces composCs utilisant des sources de lumikre monochromatique (193 ou 214 nm) provoque une ouverture de cycle qui conduit aux 1,2-bis(1-Cthylidbne)cycloalcanes (C,-C,) correspondants avec des rendements chimiques et quantiques tlevCs. Dans tous les cas, la reaction se produit avec un degrC ClevC (70-90%) de stCrCosClectivitt disrotatoire. On a determink les rendements quantiques pour la photoisomtrisation cis/trans des E,E-and E,Z-1,2-bis(1-Cthy1idkne)cycloaIcanes. Les distributions des produits dtrivant de l'irradiation des cyclobutknes dependent de la longueur d'onde, mais, pour l'excitation B 214 nm, les distributions des diknes isomkres obtenues pour l'ouverture de cycle du cyclobutkne sont en assez bon accord avec ceux calculCs ii partir des rendements quantiques pour la photoisomCrisation cis/trans des diknes isomkres en supposant que le processus implique une ouverture de cycle strictement disrotatoire pour conduire B un seul dikne isomkre dans 1'Ctat singulet excitC le plus bas. Les rtsultats sont en accord avec un mCcanisme, pour l'ouverture de cycle, qui serait adiabatique et sous contr6le d'une symCtrie des orbitales.

The effect of central bond torsional mobility on the Rydberg state ring opening of alkylcyclobutenes

“…[1]); the two E,Z-isomers are formed in a ratio that is identical (within experimental error) to that obtained from direct (254 nm) irradiation of the photochemically allowed (E,E) diene isomer (21). Reasonable (but not perfect) agreement was also observed between the isomeric diene ratios obtained from 214-nm photolysis of the symmetrically substituted bicyclic cyclobutenes 4, and values calculated from the quantum yields for E,Z-photoisomerization of their isomeric dienes 5 (19,22). This series of compounds bears the important feature that the dienes 3 and 5a-5d are constrained to exist in the s-cis conformation, similar to that which would (presumably) be formed directly in the electrocyclic ring opening reaction.…”

“…Assuming that orbital symmetry selection rules are, in fact, obeyed in the π,π* reaction, there are at least three possible explanations for this behaviour (6): (i) ring opening proceeds adiabatically, yielding the photochemically allowed (disrotatory) diene isomer(s) in the lowest π,π* excited singlet state, from which the forbidden (thermally allowed, conrotatory) isomer(s) are formed via E,Z-isomerization; (ii) the conrotatory isomers are formed via reaction from upper vibrational levels of the ground state of the cyclobutene, populated by internal conversion in competition with disrotatory π,π* state ring opening; and (iii) the conrotatory isomers are formed via competing reaction of the radical-cation-like π,R(3s) Rydberg excited singlet state, which is known to be of similar energy to the π,π* (valence) singlet state in alkylcyclobutenes (4,11,12), just as it is in other aliphatic alkenes (13)(14)(15). 2 Much of our work in this area over the past 15 years has been directed at attempting to distinguish between these possibilities, through studies of the effects of substituents, excitation wavelength, and various structural constraints on the stereochemistry of the reaction (4,6,12,(16)(17)(18)(19)(20)(21)(22)(23). By the mid-1990s, we had concluded: (i) that the π,R(3s) state is responsible for the [2 + 2]-cycloreversion reaction but is not involved in ring opening (12,24); and (ii) that the adiabatic, π,π* state ring opening mechanism most adequately accounts for the nonstereospecificity of the reaction (19,21,22).…”

“…2 Much of our work in this area over the past 15 years has been directed at attempting to distinguish between these possibilities, through studies of the effects of substituents, excitation wavelength, and various structural constraints on the stereochemistry of the reaction (4,6,12,(16)(17)(18)(19)(20)(21)(22)(23). By the mid-1990s, we had concluded: (i) that the π,R(3s) state is responsible for the [2 + 2]-cycloreversion reaction but is not involved in ring opening (12,24); and (ii) that the adiabatic, π,π* state ring opening mechanism most adequately accounts for the nonstereospecificity of the reaction (19,21,22).…”

“…The second conclusion was based on an examination of the photochemistry of the series of constrained cyclobutenes 2 and 4, comparing the relative yields of isomeric dienes obtained upon 214-nm photolysis of these compounds to values predicted from the quantum yields for direct E,Zphotoisomerization of the corresponding isomeric dienes (19,21,22). For example, 214 nm irradiation of cis-2,2,6,7tetramethylbicyclo[3.2.0]hept-1-ene (cis-2) in hydrocarbon solution yields a mixture of three diene isomers, E,E-, E,Z-, and Z,E-3 (eq.…”

“…Thus, the mechanism that emerged from these studies is one in which the π,π* excited singlet state of the cyclobutene undergoes purely disrotatory ring opening, but with internal conversion to the ground state potential energy surface occurring after the diene is fully formed rather than at a geometry intermediate between reactant and product (19,21,22); a mixture of diene isomers is formed not because the reaction itself is nonstereospecific, but because the product is formed in an excited singlet state whose main avenue for relaxation to the ground state is cis,trans-isomerization.…”